The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions.
A wide variety of battery technologies have been developed for portable and stationary applications, including lead acid, lithium-ion, nickel/metal hydride, sodium sulfur, and flow batteries, among others. (See Reference Number 1, below.) Not one of these technologies is commonly used for applications related to the stabilization and reliability of the electric grid do to exorbitantly high cost, poor cycle and calendar lifetime, and low energy efficiency during rapid cycling. However, the development of lower cost, longer lived batteries is likely needed for the grid to remain reliable in spite of the ever-increasing deployment of extremely volatile solar and wind power.
Existing battery electrode materials cannot survive for enough deep discharge cycles for the batteries containing them to be worth their price for most applications related to the electric grid. (See Reference Number 1, below.) Similarly, the batteries found in electric and hybrid electric vehicles are long lived only in the case of careful partial discharge cycling that results in heavy, large, expensive battery systems. The performance of most existing battery electrode materials during fast cycling is limited by poor kinetics for ion transport or by complicated, multi-phase operational mechanisms.
The use of Prussian Blue analogues, which are a subset of a more general class of transition metal cyanide coordination compounds (TMCCCs) of the general chemical formula AxPy[R(CN)6]z•nH2O (A=alkali cation, P and R=transition metal cations, 0≤x≤2, 0≤y≤4, 0≤z≤1, 0≤n), has been previously demonstrated as electrodes in aqueous electrolyte batteries. (See Reference Numbers 1-7.) TMCCC electrodes have longer deep discharge cycle life and higher rate capability than other intercalation mechanism electrodes, and they enjoy their highest performance in aqueous electrolytes. TMCCC cathodes rely on the electrochemical activity of iron in Fe(CN)6 complexes at high potentials. TMCCC anodes, on the other hand, contain electrochemically active, carbon-coordinated manganese or chromium.
The development of a symmetric battery in which both the anode and the cathode are each a TMCCC is desirable because TMCCCs have longer cycle life and can operate at higher charge/discharge rates than other electrode systems. If one TMCCC electrode were to be paired with a different kind of electrode, it is likely that the full battery would not last as long, or provide the same high-rate abilities as a symmetric cell containing a TMCCC anode and a TMCCC cathode.
TMCCC cathodes are well understood, and the operation of a TMCCC cathode for over 40,000 deep discharge cycles has been previously demonstrated. (See Reference Number 2.) These cathodes typically operate at about 0.9 to 1.1 V vs. the standard hydrogen electrode (SHE). (See Reference Number 8.) One challenge for the development of practical batteries using TMCCC cathodes is their trace solubility in aqueous electrolytes. Their partial dissolution into the battery electrolyte can result in a decrease in battery charge capacity due to mass loss from the electrodes and a decrease in efficiency due to side reactions with the cathode's dissolution products.
The development of a TMCCC anode has proven much more challenging than that of TMCCC cathodes because these materials typically have reaction potentials either near 0 V or below −0.5 V vs. SHE, but not in the range between −0.5 V and 0 V that is most desirable in aqueous electrolytes, and because they operate only in a narrow pH range without rapid hydrolysis to manganese dioxide phases. (See Reference Numbers 8-9.) As the useful electrochemical stability window of aqueous electrolytes at approximately neutral pH (pH=5-8) extends from about −0.4 V to 1 V vs. SHE, an anode reaction potential of 0 V results in a cell voltage lower than the maximum that is possible without decomposition of water. But, in the case of an anode reaction potential below −0.5 V vs. SHE, the charge efficiency of the anode can be poor due to rapid hydrolysis of water to hydrogen gas. Finally, if the Mn(CN)6 groups in the TMCCC anode hydrolyze, the capacity of the electrode is rapidly lost.
What is needed is a system and method for stabilizing TMCCC electrodes against dissolution and/or hydrolysis.